The nature of the interactions which exist between coenzyme and apoprotein are being studied in several flavoproteins. These include the flavodoxins and the photoreceptor protein from Euglena which functions in phototaxis. Specific projects include the following: the nature of the binding of fully-reduced flavin to apoflavodoxin will be investigated using kinetic and spectroscopic methods. Chemical modification of tyrosine, tryptophan and methionine active site residues in the flavodoxins will be used to determine the roles of these residues in catalysis. Flavin structural analogs bound to flavodoxins will be studied as a means to determine the sites of entry and exit of electrons in these proteins. An attempt will be made to develop isolation and purification methods for the photoreceptor protein of Euglena. When this has been accomplished, the nature and properties of this flavoprotein will be determined. BIBLIOGRAPHIC REFERENCES: K. Shiga and G. Tollin, "Studies on the Mechanism of Electron Transfer in Flavodoxins," Flavins and Flavoproteins, Singer, T.P. Ed., ASP, Amsterdam, 1976, p. 285. D.E. Edmondson, F. Rizzuto and G. Tollin, "Flash Photolysis Studies of 8 alpha-Substituted Flavins," Flavins and Flavoproteins, Singer, T.P. Ed., ASP, Amsterdam, 1976, p. 422.